Method to make a multilayer circuit board with intermetallic compound and related circuit boards

ABSTRACT

A method is for making a multilayer circuit board from circuit board layers, each including a dielectric layer and conductive traces thereon including a first metal. The method includes forming a through-via in a first circuit board layer, plating the through-via with the first metal, and coating a second metal onto the first metal of the first circuit board layer, the plated through-via, and the first metal. The method also includes aligning the first and second circuit board layers together so that the plated through-via of the first circuit board layer is adjacent a feature on the second circuit board layer, and heating and pressing the aligned first and second circuit board layers so as to laminate the dielectric layers together and form an intermetallic compound of the first and second metals bonding adjacent metal portions together.

FIELD OF THE INVENTION

The present invention relates to the field of multilayer circuitfabrication, and, more particularly, to lamination of multilayer circuitboards and related circuit boards.

BACKGROUND OF THE INVENTION

An electronic device may include one or more circuit boards. A typicalcircuit board is a planar board that mechanically supports electroniccomponents. The electronic components may comprise, for example,resistors, capacitors, switches, batteries, and other more complexintegrated circuit components, i.e. microprocessors. The circuit boardtypically comprises a dielectric material, for example, a polymermaterial.

The circuit board may include conductive traces on the surface forconnecting the electronic components to each other. As electroniccircuitry has become more complex, multilayer circuit boards with atleast two electrically conductive pattern layers have been developed.Typically, the different conductive trace layers of a multilayer circuitboard may be connected through vertically extending vias, which compriseconductive materials, for example, metal.

A typical multilayer circuit board may comprise a plurality of corelayers with bonding layers therebetween affixing the adjacent corelayers together. Each core layer typically includes a dielectric layerwith electrically conductive pattern layers on the opposing surfaces ofthe dielectric layer. Typically, during manufacture of the multilayercircuit boards, the core and bonding layers are stacked together andthen heated (laminated) to cause the bonding layer to affix the adjacentcore layers together.

In multilayer circuit board manufacturing, there are three typicalcategories for vias, i.e. a through-via, a buried via, and a blind-via.Quite simply, the through-via extends through all layers of a multilayercircuit board whereas a blind via terminates at one of the internallayers. Buried vias, on the other hand, connect two or more internallayers without providing connection to the external layers. In typicalfabrication approaches, the formation of the blind-via may provide sometechnical hurdles for certain plating processes. In particular, theaspect ratio for the blind-via may need to be rather wide for platingtechniques, for example, a 1:1 width to depth ratio. Accordingly, whentraversing several layers, the blind-via may become necessarily large,which may undesirably consume circuit board real estate. In contrast,through-vias may be formed with more desirable aspect ratios, forexample, 1:10 width to depth ratio.

Moreover, in typical multilayer circuit board approaches, thefabrication process includes multiple lamination steps and is onerous.Indeed, individual circuit board layers are typically laminated togethersequentially, thereby adding substantial costs to the process byrequiring registration and alignment at each sequential layer additionas well as multiple time consuming lamination steps.

For example, U.S. Patent Application No. 2008/0121416 to Hirai et al.discloses a multilayer circuit fabrication method. This method usesseparate bonding layers with through-vias to connect circuit boardlayers and uses multi-step bonding methods. Also, U.S. Pat. No.6,995,322 to Chan et al. discloses a method for making a multilayercircuit board. As in typical approaches, this method forms severalseparate portions of the finished multilayer circuit board in severallamination steps.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide an efficient method for making a multilayercircuit board.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a method of making a multilayercircuit board from a plurality of circuit board layers, each comprisinga dielectric layer and conductive traces thereon comprising a firstmetal. This method comprises forming through-via in a first circuitboard layer, plating the through-via with the first metal, and coating asecond metal onto the first metal of the first circuit board layer, thefirst metal of the plated through-via, and the first metal of a secondcircuit board layer. The method also includes aligning the first andsecond circuit board layers so that the plated through-via of the firstcircuit board layer is adjacent a feature, such as a plated through-viaor a trace, on the second circuit board layer, and heating and pressingthe aligned first and second circuit board layers so as to laminate thedielectric layers together and form an intermetallic compound of thefirst and second metals bonding adjacent metal portions together anddefining electrical connection paths of the multilayer circuit board,thereby converting the through-vias to blind or buried vias.Advantageously, the first and second circuit board layers may belaminated with smaller diameter vias in one step.

More specifically, the method may comprise selecting the second metal tohave a melting temperature below a lamination temperature of thedielectric layers. Also, the method may comprise selecting the firstmetal to have a melting temperature greater than a laminationtemperature of the dielectric layers. For example, the first metal maycomprise copper, and the second metal may comprise tin.

Another aspect is directed to a multilayer circuit board that includes aplurality of circuit board layers, each comprising a dielectric layerand conductive traces thereon comprising a first metal. The multilayercircuit board includes a first circuit board layer of the pluralitythereof having a through-via therein, and a plating in the through-viacomprising the first metal. The first circuit board layer and a secondcircuit board layer of the plurality thereof are relatively aligned sothat the through-via of the first circuit board layer is adjacent afeature on the second circuit board layer, and laminated together. Themultilayer circuit board includes adjacent portions of the conductivetraces joined together by an intermetallic compound formed between thefirst metal and a second metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method, according to the presentinvention.

FIGS. 2A-2F are schematic cross-sectional views of the steps of themethod of FIG. 1.

FIG. 3 is a phase diagram illustrating melting point characteristics foran embodiment of the intermetallic joint compound, according to thepresent invention.

FIG. 4 is a cross-sectional view of an embodiment of the intermetallicjoint compound, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIGS. 1-2F, a method of making a multilayercircuit board 40 according to the present invention is now describedwith reference to a flowchart 10. (Block 11). The method is for makingthe multilayer circuit board 40 from a plurality of circuit board layers41 a-41 c. Each circuit board layer 41 a-41 c includes a dielectriclayer 48 and first metal layers 42 on the major surfaces thereof. Thedielectric layers 48 comprise thermoplastic materials. For example, thedielectric layers 48 may include biaxially-oriented liquid crystalpolymer (LOP) layers, though other materials may be used, such aspolyimide or Teflon.

For example, the first metal layer 42 may comprise copper. Also, thefirst metal has a melting temperature greater than a laminationtemperature of the dielectric layers 48. The circuit board layers 41a-41 c are clad with the first metal on both major surfaces thereof(Block 13).

This method further includes forming a plurality of through-vias 43 in afirst circuit board layer 41 a (Block 15). For ease of explanation, onlythe first circuit board layer 41 a is shown in FIGS. 2A-E, but as willbe appreciated by those skilled in the art, the second circuit board 41b (and in certain embodiments, the third circuit board layer 41 c) isprocessed similarly. For example, the through-vias 43 may be formedusing laser milling techniques, punch techniques, and mechanicaldrilling techniques, etc.

The method includes plating the through-vias 43 with the first metal(Block 17). For example, the first metal may be plated in thethrough-vias 43 using electrodeposition methods. The method includesselectively removing portions of the first metal layers 42 to defineconductive traces 44 on the first and second circuit board layers 41a-41 b from the first metal layers thereon (Block 19). For example, thefirst metal layers 42 may be removed using chemical etching andlithography. The removal of portions of the first metal layer 42 isshown with dashed lines, since this step is optional, for example, inthe instance where a solid first metal plane is required on a specificlayer.

The method includes coating a second metal 45 onto the first metalconductive traces 44 of the first circuit board layer 41 a, the platedthrough-vias 43, and the first metal 42 of a second circuit board layer41 b (Block 21). It should be noted that the plating of the second metal45 is not drawn to scale. More specifically, the second metal 45 has amelting temperature below a lamination temperature of the dielectriclayers 48 and forms a stable intermetallic compound with the first metal42 at a temperature below the lamination temperature. The second metal45 may be formed using at least one of an electroless depositionprocess, an electrolytic deposition process, and a physical vapordeposition process. It is required that the intermetallic compound thatis formed possess a melting temperature higher than any subsequentprocessing temperatures (such as, placing components on the finishedcircuit board and performing solder reflow). For example, the secondmetal 45 may comprise tin (tin melting point: 260° C.; LCP laminationtemperature: 270° C.; copper-tin intermetallic compound melting point:600° C.).

The method also includes aligning the first and second circuit boardlayers 41 a-41 b together to interconnect the adjacent layers using theplated through-vias 43 (Block 23), thereby creating blind 49 b (FIG.2F), stacked 49 a (FIG. 2F), and/or buried vias 49 c (FIG. 2F). Themethod includes heating and applying pressure to the aligned first andsubsequent (illustratively three circuit board layers 41 a-41 c) circuitboard layers 41 a-41 c so as to laminate the dielectric layers 48together and form an intermetallic compound of the first and secondmetals bonding adjacent metal portions together and defining electricalconnection paths of the multilayer circuit board 40 (Block 29).Advantageously, the multilayer circuit board 40 can be laminated using asingle lamination cycle.

For example, in copper-tin embodiments, the intermetallic compoundcomprises an intermetallic copper-tin compound. It is preferred that thetin layer be coated onto the copper. In embodiments that use LCPdielectric layers, the aligned circuit board layers 41 a-41 c are, forexample, subjected to 270° C. and 200 PSI (lamination temperature andpressure of one type of LCP). For example, the aligned first, second,and third circuit board layers 41 a-41 c may be laminated together in anautoclave, which advantageously provides for prevention of oxidation anduses isostatic pressure, which can prevent circuit board layermis-registration and squeeze out, both of which can improve dimensionalstability.

Referring now additionally to FIGS. 3-4, for embodiments including tinas the second metal and copper as the first metal, the intermetallicjoint compound 60 comprises Cu₃Sn. Advantageously, this compound 60 isstable and has a melting point greater than 600° C. (intermetallic jointcompound melting point characteristics as tin composition varies areshown in diagram 50). The intermetallic joint compound 60 is sandwichedbetween a bottom circuit board layer 41 and a top circuit board layer 47and illustratively includes the copper conductive trace 44, a Cu₃Snlayer 48, and a copper layer 46 from the opposing copper conductivetrace. As illustrated, the heating and pressing substantially consumesthe second metal 45 by the forming of the intermetallic compound 60 withthe first metal.

In some embodiments, the method may include aligning the first circuitboard layer 41 a and the third circuit board layer 41 c on an oppositeside thereof (in relation to the second circuit board layer) (Blocks 25& 27), also shown with dashed lines, aligning and interconnecting (onboth sides of the first circuit board layer) some of the platedthrough-vias 43 of the first circuit board layer to define a buried-via49 c. In these embodiments, the third circuit board layer 41 c issimilarly formed as compared to the second and first circuit boardlayers 41 a-41 b, i.e. it comprises the dielectric layer 48, first metalconductive traces 44 thereon, and a second metal layer 45 on the firstmetal conductive traces. Of course, in these embodiments, the laminationstep comprises heating and compressing the first, second, and thirdcircuit board layers 41 a-41 c in one lamination step (Blocks 29 & 31).

Again, the above described method provides a single laminationfabrication method for the multilayer circuit board 40 that forms astable and strong intermetallic electrical interconnect. Moreover, byforming blind-vias 49 b, stacked-vias 49 a, and buried-vias 49 c frommodified through-vias, the method overcomes the typical drawbacks of viaformation in multilayer circuit boards (i.e. large blind via diametersand low aspect ratio). Being able to create all types of vias fromthrough-vias and the precision that is afforded based on one steplamination allows for greater circuit densities and more efficient useof circuit board real estate.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A method of making a multilayer circuitboard from a plurality of circuit board layers, each comprising adielectric layer and conductive traces thereon comprising a first metal,the method comprising: forming a through-via in a first circuit boardlayer; plating the through-via with the first metal; selecting a secondmetal to have a melting temperature below a lamination temperature ofthe dielectric layers; coating the second metal onto the first metal ofthe first circuit board layer, the plated through-via, and the firstmetal of a second circuit board layer; aligning the first and secondcircuit board layers together so that the plated through-via of thefirst circuit board layer is adjacent a feature on the second circuitboard layer; and heating and pressing the aligned first and secondcircuit board layers so as to directly laminate the dielectric layerstogether without using adhesive material, and form a joint comprising anintermetallic compound of the first and second metals bonding adjacentmetal portions together and defining electrical connection paths of themultilayer circuit board, the intermetallic compound having a meltingtemperature above the lamination temperature of the dielectric layers.2. The method according to claim 1 further comprising selecting thefirst metal to have a melting temperature greater than a laminationtemperature of the dielectric layers.
 3. The method according to claim 1wherein the first metal comprises copper; and wherein the second metalcomprises tin.
 4. The method according to claim 1 further comprising:coating the second metal onto the first metal of a third circuit boardlayer; and aligning the first, the second, and the third circuit boardlayers to each other so that the plated through-via of the first circuitboard layer is aligned to the feature on the second circuit board layerand a plated through-via of the second circuit board layer is aligned toa feature on the third circuit board layer; and wherein heating andpressing comprises heating and pressing the first, second, and thirdcircuit board layers.
 5. The method according to claim 1 furthercomprising: forming a through-via in the second circuit board layer;plating the through-via with the first metal; and coating the secondmetal onto the plated through-via of the second circuit board.
 6. Themethod according to claim 1 wherein the dielectric layers comprisethermoplastic materials.
 7. The method according to claim 1 furthercomprising selectively removing portions of the first metal to definethe conductive traces on the first and second circuit board layers. 8.The method according to claim 1 wherein heating and pressingsubstantially consumes the second metal by the forming of theintermetallic compound with the first metal.
 9. The method according toclaim 1 wherein forming the through-via comprises at least one of lasermilling, drilling, and punching the through-via.
 10. A method of makinga multilayer circuit board from a plurality of circuit board layers,each comprising a thermoplastic dielectric layer and conductive tracesthereon comprising copper material, the method comprising: forming athrough-via in a first circuit board layer; plating the through-via withthe copper material; selecting a tin material to have a meltingtemperature below a lamination temperature of the thermoplasticdielectric layers; coating the tin material onto the copper material ofthe first circuit board layer, the plated through-via, and the coppermaterial of a second circuit board layer; aligning the first and secondcircuit board layers together so that the plated through-via of thefirst circuit board layer is adjacent a feature on the second circuitboard layer; and heating and pressing the aligned first and secondcircuit board layers so as to directly laminate the thermoplasticdielectric layers together without using adhesive material, and form ajoint comprising an intermetallic copper-tin compound bonding adjacentmetal portions together and defining electrical connection paths of themultilayer circuit board, the intermetallic copper-tin compound having amelting temperature above the lamination temperature of thethermoplastic dielectric layers.
 11. The method according to claim 10wherein heating and pressing the aligned first and second circuit boardlayers for such a time that allows for formation of the intermetalliccopper-tin compound.
 12. The method according to claim 10 furthercomprising deposited the tin material through at least one of anelectroless deposition process, an electrolytic deposition process, anda physical vapor deposition process.
 13. The method according to claim10 further comprising selecting the thermoplastic dielectric layers tohave a lamination temperature lower than a melting temperature of thecopper material.
 14. The method according to claim 10 furthercomprising: coating the tin material onto the copper material of a thirdcircuit board layer; and aligning the first, the second, and the thirdcircuit board layers to each other so that the plated through-via of thefirst circuit board layer is aligned to the feature on the secondcircuit board layer and a plated through-via of the second circuit boardlayer is aligned to a feature on the third circuit board layer; andwherein heating and pressing comprises heating and pressing the first,second, and third circuit board layers.
 15. The method according toclaim 10 further comprising: forming a through-via in the second circuitboard layer; plating the through-via with the copper material; andcoating the tin material onto the plated through-via of the secondcircuit board.
 16. The method according to claim 10 further comprisingselectively removing portions of the copper material to define theconductive traces on the first and second circuit board layers.